CN120603974A

CN120603974A - Seamless steel pipe and airbag inflator cylinder

Info

Publication number: CN120603974A
Application number: CN202480008534.0A
Authority: CN
Inventors: 青木孝辅; 中村浩史
Original assignee: Nippon Steel and Sumitomo Metal Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-03-09
Filing date: 2024-02-19
Publication date: 2025-09-05
Also published as: WO2024185477A1; EP4678775A1; JP7674693B2; AR131957A1; MX2025007372A; JPWO2024185477A1

Abstract

一种无缝钢管，其具有说明书中记载的化学组成，该化学组成与壁厚WT(mm)的关系满足[WT/(5C+Mo+Cr)≥1.00]，与原γ晶粒度GN的关系满足[GN‑1.51×(Mn+85P‑30Ca)≥8.50]，所述无缝钢管的拉伸强度为1000MPa以上，断裂伸长率为8.0％以上，极限氢浓度为2.5ppm以上。A seamless steel pipe having a chemical composition described in the specification, wherein the relationship between the chemical composition and the wall thickness WT (mm) satisfies [WT/(5C+Mo+Cr)≥1.00], and the relationship between the chemical composition and the original γ grain size GN satisfies [GN-1.51×(Mn+85P-30Ca)≥8.50]. The seamless steel pipe has a tensile strength of 1000 MPa or greater, an elongation at break of 8.0% or greater, and a limiting hydrogen concentration of 2.5 ppm or greater.

Description

Air bottle for seamless steel tube and safety air bag

Technical Field

The present invention relates to a seamless steel pipe and an inflatable bottle for an airbag.

Background

Devices for which safety is required are actively introduced in the automotive industry. Among them, an airbag system is mounted, and in the event of a collision, the airbag is deployed between a steering wheel, an instrument panel, and the like by means of a gas or the like before the occupant collides with the vehicle, and kinetic energy of the occupant is absorbed to reduce injuries. As an airbag system, a method using an explosive drug has been conventionally employed, but from the viewpoint of environmental recovery, a system using a high-pressure filling gas has been developed and widely used.

The above system always holds the gas or the like blown into the airbag at the time of collision at a high pressure, and ejects the gas at a burst at the time of collision. This causes the steel pipe for the airbag to be subjected to stress at a high strain rate in a very short time. Therefore, the steel pipe used is required to have high strength and excellent blast resistance.

Recently, the demand for weight reduction of automobiles is increasing. From this point of view, the steel tube for vehicle-mounted airbags is also required to be thin and lightweight, and in order to ensure a high burst pressure even when it is thin, an inflatable bottle made of a high-strength seamless steel tube having a tensile strength of 900MPa or more is used in the airbag system.

Further, for example, in the production of a gas cylinder or the like, since the diameter reduction process is performed, excellent diameter reduction workability is required for the steel pipe for an airbag.

Against this background, for example, patent documents 1 and 2 disclose seamless steel pipes for airbags. According to patent documents 1 and 2, studies have been made on improving strength and toughness and workability.

Prior art literature

Patent literature

Patent document 1 Japanese patent application laid-open No. 2004-27303

Patent document 2 Japanese patent application laid-open No. 2004-76034

Disclosure of Invention

Problems to be solved by the invention

However, as the steel pipe for an airbag, in order to secure higher reliability, it is required to suppress embrittlement caused by hydrogen that intrudes into the steel pipe in the manufacturing process and the use environment. Patent documents 1 and 2 do not at all investigate the hydrogen embrittlement resistance of steel pipes.

The purpose of the present invention is to provide a seamless steel pipe and an airbag inflation bottle that have high strength, excellent diameter reduction workability, and excellent hydrogen embrittlement resistance.

Solution for solving the problem

The present invention has been made to solve the above-described problems, and its gist is a seamless steel tube and an airbag inflation bottle as described below.

(1) A seamless steel pipe, which is made of a metal material,

Its chemical composition is in mass%

C:0.05~0.20%、

Si:0.05~0.50%、

Mn:0.30~1.50%、

P is below 0.025%,

S is less than 0.020%,

Cu:0.01~0.50%、

Ni:0.01~0.50%、

Cr:0.01~1.20%、

Mo:0.01~0.50%、

Ti:0.001~0.050%、

Nb:0.001~0.100%、

Ca:0.0005~0.0025%、

Al of 0.080% or less,

N is less than 0.0100%,

V:0~0.100%、

B:0~0.0050%、

Mg:0~0.0050%、

REM:0~0.0050%、

Sn:0~0.100%、

As:0~0.010%、

The balance of Fe and impurities,

On the premise that the content of each element is within the above range,

The relationship between the chemical composition and the wall thickness satisfies the following formula (i),

The relationship between the chemical composition and the prior austenite grain size satisfies the following formula (ii),

The tensile strength is more than 1000MPa,

The elongation at break is more than 8.0 percent,

The limiting hydrogen concentration is more than 2.5 ppm.

WT/(5C+Mo+Cr)≥1.00···(i)

GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

Wherein the symbol of the element in the above formula represents the content (mass%) of each element in the steel, and the symbol is zero when the steel is not contained. WT represents the wall thickness (mm) of the seamless steel pipe, and GN represents the prior austenite grain size.

(2) The seamless steel pipe according to the above (1), wherein,

The chemical composition contains, in mass%, a component selected from the group consisting of

V:0.001~0.100%、

B:0.0001~0.0050%、

Mg:0.0001~0.0100%、

REM:0.0001~0.0100%、

0.001 To 0.100% Sn, and

0.001 To 0.010% of As.

(3) An airbag inflation bottle comprising:

a cylindrical portion extending in one direction, and a reduced diameter portion formed on at least one end side of the cylindrical portion in the one direction,

The chemical composition of the cylindrical portion is as mass percent

C:0.05~0.20%、

Si:0.05~0.50%、

Mn:0.30~1.50%、

P is below 0.025%,

S is less than 0.020%,

Cu:0.01~0.50%、

Ni:0.01~0.50%、

Cr:0.01~1.20%、

Mo:0.01~0.50%、

Ti:0.001~0.050%、

Nb:0.001~0.100%、

Ca:0.0005~0.0025%、

Al of 0.080% or less,

N is less than 0.0100%,

V:0~0.100%、

B:0~0.0050%、

Mg:0~0.0050%、

REM:0~0.0050%、

Sn:0~0.100%、

As:0~0.010%、

The balance of Fe and impurities,

On the premise that the content of each element is within the above range,

The relationship between the chemical composition of the cylindrical portion and the wall thickness of the cylindrical portion satisfies the following formula (i),

The relationship between the chemical composition of the cylindrical portion and the prior austenite grain size of the cylindrical portion satisfies the following formula (ii),

The tensile strength of the cylindrical portion is 1000MPa or more,

The elongation at break of the cylindrical portion is 8.0% or more,

The limiting hydrogen concentration of the cylindrical portion is 2.5ppm or more.

WT/(5C+Mo+Cr)≥1.00···(i)

GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

The symbol of the element in the above formula represents the content (mass%) of the steel of each element in the cylindrical portion, and the symbol is zero when the steel is not contained. WT represents the wall thickness (mm) of the cylindrical portion, and GN represents the prior austenite grain size of the cylindrical portion.

(4) The airbag inflation bottle according to the above (3), wherein,

The chemical composition of the cylindrical portion is selected from the group consisting of

V:0.001~0.100%、

B:0.0001~0.0050%、

Mg:0.0001~0.0100%、

REM:0.0001~0.0100%、

0.001 To 0.100% Sn, and

0.001 To 0.010% of As.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a seamless steel pipe and an airbag inflation bottle having high strength, excellent diameter reduction workability, and excellent hydrogen embrittlement resistance can be obtained.

Drawings

FIG. 1 is a diagram for explaining the shape of an arc tensile test piece used for measuring the limiting hydrogen concentration.

Detailed Description

The present inventors have conducted intensive studies on a method required to ensure hydrogen embrittlement resistance while simultaneously achieving strength and reducing workability of a seamless steel pipe. As a result, the following findings were obtained.

(A) In order to increase the strength of the seamless steel pipe, the content of elements that can improve the hardenability needs to be increased. Wherein, the content of C, mo and Cr is sufficiently ensured to be effective. However, if the content of these elements is excessive in relation to the wall thickness of the seamless steel pipe, the reducing workability cannot be ensured. From this viewpoint, it is important to control the balance between the chemical composition and the wall thickness of the seamless steel pipe, specifically, to satisfy the following expression (i).

WT/(5C+Mo+Cr)≥1.00···(i)

(B) In order to secure the reducing workability and to improve the ductility, it is also important to make the elongation at break 8.0% or more. In addition to controlling the chemical composition, the elongation at break can be achieved by manufacturing under appropriate conditions.

(C) When the Mn content is excessive, not only the diffusion rate of hydrogen is reduced, localized enrichment occurs, but MnS is also generated, resulting in deterioration of hydrogen embrittlement resistance. In addition, P segregates at grain boundaries, degrading hydrogen embrittlement resistance. On the other hand, ca has an effect of suppressing the formation of MnS, and thus can improve the hydrogen embrittlement resistance.

(D) The inventors of the present invention have studied and found that the degree of deterioration of hydrogen embrittlement resistance varies depending on the prior austenite grain size. Further, as a result of evaluating the effects of the contents of Mn, P, and Ca and the prior austenite grain size GN on the hydrogen embrittlement resistance, it was found that excellent hydrogen embrittlement resistance can be obtained by adjusting the contents of the respective elements to fall within a predetermined range and satisfying the following formula (ii).

GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

(E) To further improve the hydrogen embrittlement resistance, preheating is required in the tempering step. Although the mechanism of further improving the hydrogen embrittlement resistance by preheating is not clear, it is considered that the temperature distribution in the wall thickness direction is eliminated, and the metallographic structure becomes uniform.

The present invention has been made based on the above-described findings. The following describes each configuration of the present invention in detail.

(A) Chemical composition

The reason why the chemical composition of the seamless steel pipe according to one embodiment of the present invention is limited is as follows. In the following description, "%" for each element content means "% by mass".

C:0.05~0.20%

C is an effective element for enhancing the strength of steel at low cost. If the content is less than 0.05%, it is difficult to obtain a desired tensile strength, and if it exceeds 0.20%, the workability and weldability are lowered. Therefore, the content of C is set to 0.05 to 0.20%. The preferable range of the C content is 0.06% or more and 0.18% or less, and the more preferable range is 0.07% or more and 0.17% or less. In the case where the reducing workability is particularly important, the C content is more preferably less than 0.17%.

Si:0.05~0.50%

Si is an element that can enhance the hardenability of steel and strength in addition to deoxidization. For this purpose, the content of Si is set to 0.05% or more. However, if the content exceeds 0.50%, the toughness is lowered, and therefore the content of Si is set to 0.50% or less. The preferable range of the Si content is 0.10% or more and 0.40% or less, and the more preferable range is 0.15% or more and 0.30% or less.

Mn:0.30~1.50%

Mn is an effective element that can enhance the hardenability of steel to improve strength and toughness in addition to deoxidization. However, the content of less than 0.30% does not give sufficient strength and toughness. On the other hand, when the Mn content exceeds 1.50%, mnS coarsens, and the MnS expands during hot rolling, and the toughness and hydrogen embrittlement resistance are lowered. Further, excessive Mn reduces the diffusion rate of hydrogen, and local enrichment occurs, thereby reducing the hydrogen embrittlement resistance. Therefore, the Mn content is set to 0.30 to 1.50%. The Mn content is preferably 0.40% or more and 1.20% or less, and more preferably in the range of 0.50% or more and 1.00% or less.

P is less than 0.025%

P is contained in the steel in the form of impurities, which results in a decrease in toughness and hydrogen embrittlement resistance caused by grain boundary segregation. In particular, if the content of P exceeds 0.025%, the decrease in toughness and hydrogen embrittlement resistance becomes remarkable. Therefore, the content of P is set to 0.025% or less. The content of P is preferably 0.020% or less, more preferably 0.015% or less.

S is less than 0.020%

S is also contained in the steel in the form of impurities, and in particular, decreases the toughness of the steel pipe in the T direction (direction orthogonal to the pipe axis direction of the steel pipe). If the S content exceeds 0.020%, the toughness in the T direction of the steel pipe becomes remarkable, and therefore the S content is set to 0.020% or less. The S content is preferably 0.010% or less.

Cu:0.01~0.50%

Cu enhances strength and toughness by enhancing hardenability of steel. The effect is exhibited when Cu is contained in an amount of 0.01% or more. However, if Cu is contained in an amount exceeding 0.50%, the alloy cost increases. Therefore, the Cu content is set to 0.01-0.50%. The Cu content is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.20% or more. The Cu content is preferably 0.40% or less, more preferably 0.35% or less.

Ni:0.01~0.50%

Ni enhances hardenability of steel, thereby improving strength and toughness. The effect is exhibited when Ni is contained in an amount of 0.01% or more. However, when Ni is contained in an amount exceeding 0.50%, the alloy cost increases, and therefore the Ni content is set to 0.01 to 0.50%. The Ni content is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.20% or more. The Ni content is preferably 0.45% or less, more preferably 0.40% or less.

Cr:0.01~1.20%

Cr enhances the hardenability of steel, enhances the tempering softening resistance and improves the strength and toughness. This effect is exhibited when Cr is contained in an amount of 0.01% or more. However, if Cr is contained in an amount exceeding 1.20%, the alloy cost increases. Therefore, the Cr content is set to 0.01 to 1.20%. The Cr content is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.20% or more. The Cr content is preferably 1.00% or less, more preferably 0.90% or less.

Mo:0.01~0.50%

Mo enhances the hardenability of steel, enhances the tempering softening resistance and improves the strength and toughness. The effect is exhibited when Mo is contained at 0.01% or more. However, if Mo is contained in an amount exceeding 0.50%, the alloy cost increases. If the Mo content is too high, the strength tends to be high even in the air cooling after the hot working of the seamless steel pipe, and softening heat treatment is required before cold drawing, resulting in an increase in manufacturing cost. Therefore, the content of Mo is set to 0.01-0.50%. The Mo content is preferably 0.05% or more, more preferably 0.10% or more, and still more preferably 0.20% or more. The Mo content is preferably 0.45% or less, more preferably 0.40% or less.

Ti:0.001~0.050%

Ti fixes N in steel, improving toughness. In addition, the finely dispersed Ti nitrides can strongly pin grain boundaries, refine grains and improve the toughness of steel. When the content exceeds 0.050%, the nitrides coarsen and the toughness is rather lowered. Therefore, the Ti content is set to 0.001 to 0.050%. The Ti content is preferably 0.003% or more, more preferably 0.005% or more, and still more preferably 0.010% or more. The Ti content is preferably 0.045% or less, more preferably 0.040% or less, and still more preferably 0.030% or less.

Nb:0.001~0.100%

Nb is finely dispersed in the steel in the form of carbides, strongly pinning grain boundaries. This has the effect of refining the crystal grains and improving the toughness of the steel. When the content exceeds 0.100%, the carbide coarsens and the toughness is rather lowered. Therefore, the Nb content is set to 0.001 to 0.100%. The Nb content is preferably 0.005% or more, more preferably 0.010% or more, and still more preferably 0.015% or more. The Nb content is preferably 0.080% or less, more preferably 0.060% or less.

Ca:0.0005~0.0025%

Ca fixes S, which exists as an unavoidable impurity in steel, as sulfide, improves anisotropy of toughness, and increases toughness in T direction of steel pipe, thereby enhancing blast resistance. Further, suppression of MnS production contributes to improvement of hydrogen embrittlement resistance. This effect is exhibited when Ca is contained in an amount of 0.0005% or more. However, if the content exceeds 0.0025%, the number of inclusions increases, and the toughness decreases. Therefore, the Ca content is set to 0.0005 to 0.0025%. In order to reliably obtain the effect of improving the hydrogen embrittlement resistance, the Ca content is preferably 0.0010% or more, more preferably more than 0.0010%, still more preferably 0.0012% or more, and still more preferably 0.0015% or more.

Al of 0.080% or less

Al is an effective element having deoxidizing effect and enhancing toughness and workability. However, if the content exceeds 0.080%, the generation of macro streaks (macro-streak-flaw) becomes remarkable. Therefore, the content of Al is set to 0.080% or less. The Al content is preferably 0.060% or less, more preferably 0.040% or less. The lower limit of the Al content is not particularly limited, but is preferably 0.005% or more, since the Al content may be an impurity level. The Al content in the present invention means the content of acid-soluble Al (so-called "sol.al").

N is less than 0.0100%

N forms fine nitrides, thereby strongly pinning grain boundaries, refining grains, and improving toughness of steel. However, if the content exceeds 0.0100%, the nitride coarsens, and the toughness is rather lowered. Therefore, the content of N is set to 0.0100% or less. The N content is preferably 0.0080% or less, more preferably 0.0050% or less. The lower limit of the N content is not particularly limited, but is preferably 0.0005% or more, more preferably 0.0010% or more, since the N content may be an impurity level.

V:0~0.100%

V is an element that secures toughness and enhances strength by precipitation strengthening, and may be contained as needed. However, if the content exceeds 0.100%, toughness is lowered. Therefore, the V content is set to 0.100% or less when contained. The V content is preferably 0.050% or less, more preferably 0.010% or less. Even in a small amount, the effect of V can be confirmed, but in order to obtain a sufficient effect, it is preferable to contain 0.001% or more.

B:0~0.0050%

B is an element that is added in a trace amount to cause grain boundary segregation in steel and significantly improve hardenability of steel, and thus may be contained as needed. However, when the content of B exceeds 0.0050%, coarse precipitation of boride at grain boundaries and a decrease in toughness are observed. Therefore, the content of B is set to 0.0050% or less when contained. The B content is preferably 0.0030% or less, more preferably 0.0020% or less. The effect of B can be confirmed even in a small amount, but in order to ensure a sufficient effect, it is preferably 0.0001% or more, more preferably 0.0005% or more.

Mg:0~0.0050%。

Like Ca, mg is an element that fixes S, which exists as an unavoidable impurity in steel, as sulfide, improves anisotropy of toughness, enhances toughness in the T direction of a steel pipe, and thereby enhances blast resistance, and thus may be contained as needed. However, if the content exceeds 0.0050%, the number of inclusions increases, and the toughness decreases. Therefore, the Mg content is set to 0.0050% or less when contained. The Mg content is preferably 0.0040% or less, more preferably 0.0030% or less. The effect of Mg can be confirmed even in a small amount, but in order to ensure a sufficient effect, it is preferably 0.0001% or more, more preferably 0.0005% or more.

REM:0~0.0050%

Like Ca, REM is an element that fixes S, which exists as an unavoidable impurity in steel, as sulfide, improves anisotropy of toughness, enhances toughness in the T direction of a steel pipe, and thus enhances blast resistance, and thus may be contained as needed. However, if the content exceeds 0.0050%, the number of inclusions increases, and the toughness decreases. Therefore, when the REM content is contained, the REM content is set to 0.0050% or less. The REM content is preferably 0.0040% or less, more preferably 0.0030% or less. The REM effect can be confirmed even in a small amount, but in order to ensure a sufficient effect, it is preferably 0.0001% or more, more preferably 0.0005% or more.

In the present embodiment, "REM" means 17 elements in total of Sc, Y and lanthanoid, and "REM content" means the content when REM is 1, and the total content when REM is 2 or more. In addition, REM is also generally supplied in the form of a misch metal, which is an alloy of a plurality of REMs. Thus, it is possible to add and contain 1 or 2 or more individual elements, for example, or to add them in the form of mixed rare earth metals.

Sn:0~0.100%

Since Sn has an effect of improving corrosion resistance, sn may be contained as needed. However, if Sn is excessively contained, toughness is lowered. Therefore, the Sn content is 0.100% or less. The Sn content is preferably 0.080% or less, more preferably 0.060% or less. Although the effect of Sn can be confirmed even in a small amount, it is preferably 0.001% or more, more preferably 0.003% or more, in order to ensure a sufficient effect.

As:0~0.010%

As has an effect of improving corrosion resistance, and therefore may be contained As needed. However, if As is contained excessively, the hot workability is deteriorated. Therefore, the As content is 0.010% or less. The As content is preferably 0.008% or less, more preferably 0.006% or less. Even in a small amount, the effect of As can be confirmed, but in order to ensure a sufficient effect, it is preferably 0.001% or more, more preferably 0.002% or more.

The seamless steel pipe according to the present embodiment contains the above elements, and the balance is Fe and impurities. Here, "impurities" are components mixed in by various causes of raw materials such as ores and scraps in the industrial production of steel materials, and are allowed within a range that does not adversely affect the present invention.

The chemical composition of the seamless steel pipe according to the present embodiment satisfies the following expression (i) with respect to the relationship between the element content and the wall thickness within the above-described range. As described above, by sufficiently securing C, mo and Cr contents, hardenability improves, and strength of the seamless steel pipe can be increased. However, from the viewpoint of reducing workability, it is necessary to adjust the balance between the chemical composition and the wall thickness of the seamless steel pipe. By satisfying the following expression (i), the reducing workability can be ensured. The left-hand value of the following formula (i) is preferably 1.20 or more, more preferably 1.50 or more, and still more preferably 2.00 or more.

WT/(5C+Mo+Cr)≥1.00···(i)

Wherein the symbol of the element in the above formula represents the content (mass%) of each element in the steel, and the symbol is zero when the steel is not contained. Further, WT represents the wall thickness (mm) of the seamless steel pipe.

In addition, the chemical composition of the seamless steel pipe according to the present embodiment satisfies the following expression (ii) in relation to the prior austenite grain size on the premise that the content of each element is within the above-described range. The Mn and P that deteriorate the hydrogen embrittlement resistance and the Ca content that improves the hydrogen embrittlement resistance are adjusted according to the prior austenite grain size, whereby excellent hydrogen embrittlement resistance can be obtained. The left-hand value of the following formula (ii) is preferably 9.00 or more, more preferably 9.50 or more, and still more preferably 10.00 or more.

GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

Wherein the symbol of the element in the above formula represents the content (mass%) of each element in the steel, and the symbol is zero when the steel is not contained. GN represents the prior austenite grain size.

The prior austenite grain size is determined in accordance with ASTM E112 (2013). Specifically, a test piece including the entire wall thickness was collected so that the surface of the seamless steel pipe including the pipe axis direction and the wall thickness direction (hereinafter also referred to as "longitudinal section") was the surface to be detected (hereinafter also referred to as "observation surface"), and mirror polishing of the observation surface was performed. After polishing, the prior austenite grain boundaries in the observation plane were developed using picric acid etching solution.

Then, 5 visual fields were observed using an optical microscope so that the position 1/4 of the wall thickness from the outer surface of the seamless steel pipe was the center of the visual field. Then, the prior austenite grain size of each field was determined by a comparison method defined in ASTM E112 (2013), and the average value thereof was used as the prior austenite grain size of the seamless steel pipe. At this time, 100 times is taken as a reference observation magnification, and 200 times or 400 times is set depending on the grain size. When the observation magnification is 200 times or 400 times, correction is performed according to ASTM E112 (2013) using a correction value Q defined by the following formula (I).

Q=6.64log₁₀(M/100)···(I)

Wherein M in the above formula is the observation magnification.

The prior austenite grain size is not particularly limited as long as the above formula (ii) is satisfied, and may be, for example, 10.0 or more or 11.0 or more.

(B) Wall thickness

The thickness of the seamless steel pipe according to the present embodiment is not particularly limited as long as the above formula (i) is satisfied, and may be, for example, 1.00mm or more or 1.50mm or more. On the other hand, the thickness is preferably thin from the viewpoint of weight reduction, and the wall thickness is preferably 2.60mm or less, more preferably less than 2.50mm, and still more preferably 2.40mm or less. In general, the thinner the wall thickness is, the more difficult the diameter reduction process is. However, in the present invention, by adjusting the balance between the chemical composition and the wall thickness of the seamless steel pipe, the reducing workability can be ensured even if the steel pipe is thin.

(C) Characteristics of

The seamless steel pipe according to the present embodiment has high strength, specifically, a tensile strength of 1000MPa or more. When the tensile strength is 1000MPa or more, excellent burst resistance can be exhibited even when the airbag bottle is used as an airbag bottle that is subjected to a stress at a large strain rate in a very short time.

In order to increase the strength, the content of the element for improving the hardenability needs to be further increased as described above, and as a result, the risk of reducing workability increases. In the case where the diameter reduction workability is important, the tensile strength is preferably less than 1200MPa.

Further, the seamless steel pipe according to the present embodiment has excellent ductility, specifically, an elongation at break of 8.0% or more, because of securing the reducing workability. The elongation at break is preferably 9.0% or more, more preferably 10.0% or more.

Tensile strength and elongation at break were measured in accordance with JIS Z2241:2011. Specifically, a tubular test piece of a predetermined length was cut out of a seamless steel pipe, and a test piece No. 11 was produced in accordance with JIS Z2241:2011. Then, a tubular tensile test defined in JIS Z2241:2011 was performed using the test piece No. 11, whereby tensile strength and elongation at break were measured.

Further, the seamless steel pipe according to the present embodiment has excellent hydrogen embrittlement resistance, and specifically has a limiting hydrogen concentration of 2.5ppm or more. This ensures high reliability when used as a steel tube for an airbag or the like. The limiting hydrogen concentration is more preferably 2.7ppm or more. In the present embodiment, the limiting hydrogen concentration is specifically determined by the following method.

A plurality of arc tensile test pieces of the shape shown in fig. 1 were collected from a seamless steel pipe. The arc tensile test piece was produced by cutting an arc test piece having a length of 120mm, a width of 9.0mm and a thickness of the original wall thickness d of the steel pipe from the seamless steel pipe, and then providing a width-reduced portion at the center in the longitudinal direction and further providing a U-cut at the center in the longitudinal direction of the width-reduced portion with holding portions left at both ends in the longitudinal direction. The holding parts are respectively 45mm long and 9.0mm wide, and the length of the width shrinking part is 30mm and the width is 2.0mm. Further, both ends of the tenter portion are formed into curved surfaces having a radius of curvature of 5.0mm, and are connected to the grip portion. The U-shaped incision is 0.20mm in incision width, 0.35mm in incision depth and 0.10mm in incision bottom radius.

Next, a cathode charge constant load test was performed at a potential ranging from-0.9 to-1.2V while immersing a plurality of arc tensile test pieces in various aqueous solutions containing 3% NaCl and ammonium thiocyanate in a range of 0 to 30 g/L. At this time, a stress of 90% of the tensile strength of each seamless steel pipe was applied.

Then, only the arc tensile test piece having a endurance time exceeding 200 hours was stored in liquid nitrogen, and then the parallel portion of the reduced width portion was cut to prepare a test piece for measuring hydrogen concentration, and the hydrogen concentration was measured by a temperature programmed desorption hydrogen analysis method. In the temperature programmed desorption hydrogen analysis method, a hydrogen concentration measurement test piece is heated from normal temperature to 200 ℃ at a temperature rise rate of 100 ℃ per hour, and then the amount of hydrogen released is measured, thereby obtaining the hydrogen concentration in the test piece. The maximum value of the obtained hydrogen concentrations was set as the limiting hydrogen concentration.

(D) Inflatable bottle for safety air bag

An airbag inflation bottle according to one embodiment of the present invention includes a cylindrical portion extending in one direction and a reduced diameter portion formed on at least one end side of the cylindrical portion in one direction. The reduced diameter portion may be formed on both end sides of the cylindrical portion in one direction.

The airbag inflation bottle according to the present embodiment is manufactured by cutting the seamless steel pipe to a predetermined length and then reducing one or both ends thereof. Therefore, the chemical composition, prior austenite grain size, wall thickness, and properties of the cylindrical portion are the same as those of the seamless steel pipe as a blank. Therefore, the description thereof is omitted.

The reduced diameter portion is reduced in diameter, and the strength and wall thickness are equal to or higher than those of a seamless steel pipe as a blank. That is, if a seamless steel pipe as a blank has high strength and excellent hydrogen embrittlement resistance, an airbag inflation bottle manufactured from the seamless steel pipe can also have high strength and excellent hydrogen embrittlement resistance.

(E) Method of manufacture

The seamless steel pipe according to an embodiment of the present invention can be manufactured by the following method.

Steel having the chemical composition described in the foregoing (a) is melted by a usual method, and then cast into an ingot or a billet by casting. The cast slab having a round strand shape for pipe production may be produced by the so-called "round strand CC" (round strand continuous casting, round Continuous Casting) method.

As a next step, the cast ingot or cast billet is subjected to blooming or hot forging. This step is a step of obtaining a billet used for final hot working of a pipe (for example, pipe production by hot piercing, rolling and drawing steps, or pipe production by hot extrusion). In addition, since a billet having a round billet shape obtained by the "round billet CC" method can be directly processed into a seamless steel pipe, it is not necessarily required to perform blooming or hot forging.

The blank for use in the final heat-treated pipe manufactured by the above-described blooming or hot forging, or the cast blank (hereinafter, these will be collectively referred to as "billet") formed into a round billet shape is subjected to a heat-treated pipe manufacturing process, a cold working process, a quenching process, and a tempering process in this order, whereby a seamless steel pipe according to the present embodiment is manufactured.

< Hot working tubing Process >

The billet is heated and then heat-processed to produce a tube, thereby producing a tube blank having a predetermined shape. As the hot working pipe making method, a conventional method can be used, and for example, a mannesman mandrel mill pipe method can be employed. The heating temperature of the billet may be set to, for example, 1000 to 1300 ℃.

< Cold working procedure >

The pipe blank obtained by the above method is subjected to cold working in order to improve dimensional accuracy. The cold working method is not particularly limited as long as it is a method capable of uniformly working a pipe blank, and for example, it is industrially advantageous to use a so-called cold drawing machine using a piercing die and a plug, a cold rolling mill called a cold rolling mill, or the like.

< Quenching Process >

Then, the cold-worked pipe blank is subjected to induction hardening treatment, and the induction hardening treatment is quenched after being heated to a temperature of 900 to 1050 ℃ at high frequency. If the heating temperature is less than 900 ℃, austenitization cannot be completed, and thus high strength may not be provided. On the other hand, if the heating temperature exceeds 1050 ℃, austenite grains grow rapidly and become coarse, and thus cannot have excellent toughness.

In addition, by rapid heating by high-frequency heating, the growth of austenite grains can be suppressed, and a fine metallographic structure can be obtained. Although the holding time at the heating temperature is also dependent on the size of the tube blank, it is preferably 10 seconds or less from the viewpoint of suppressing the growth of austenite grains. The heating temperature refers to the temperature of the outer surface of the tube blank. In quenching, if a sufficient quenched structure can be obtained, a suitable method such as water cooling or oil cooling may be used.

< Tempering Process >

And (3) tempering the tube blank after high-frequency quenching, wherein the tube blank is heated to 370-410 ℃ and then cooled to room temperature. If the heating temperature of tempering is lower than 370 ℃, the ductility and low-temperature toughness are reduced although the strength can be ensured. In particular, when the ductility is lowered, sufficient reducing workability cannot be ensured even if the above formula (i) is satisfied. On the other hand, if the heating temperature of tempering exceeds 410 ℃, even if excellent ductility and low-temperature toughness can be obtained, the strength is lowered, and a tensile strength of 1000MPa or more cannot be obtained.

The holding time at the heating temperature is preferably 10 to 30 minutes, although it depends on the size of the tube blank. The heating temperature refers to the temperature of the outer surface of the tube blank. The cooling rate at the time of tempering is not particularly limited. Therefore, the cooling in the air, such as natural cooling, forced air cooling, spray cooling, oil cooling, and water cooling, may be performed in accordance with the equipment.

In order to obtain excellent hydrogen embrittlement resistance, it is necessary to preheat the sheet at a stage before the temperature rises to the heating temperature. Specifically, the preheating is performed so that the residence time in the temperature range of 250 to 350 ℃ is 5 minutes or longer. As described above, it is considered that by preheating, the temperature distribution in the wall thickness direction is eliminated, and the metallographic structure becomes uniform.

As described above, the airbag inflation bottle according to the present embodiment is manufactured by cutting a seamless steel pipe manufactured by the above method to a predetermined length and then reducing one or both ends of the cut seamless steel pipe. The cutting and diameter reduction may be performed by a known method.

Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to these examples.

Examples

Steel having the chemical composition shown in tables 1-1 and 1-2 was melted and rectangular bar was cast using a converter-continuous casting process. The rectangular bar is further formed into a round bar by hot forging and cooled to room temperature.

[ Table 1-1]

TABLE 1-1

[ Tables 1-2]

TABLE 1-2

The round billet is heated, a tube blank is manufactured by a Mannesmann mandrel rolling tube mode, and the tube blank is cooled to room temperature. The resulting tube blank was cold worked using a cold drawing machine to achieve the outer diameter and wall thickness shown in table 2. Then, quenching and tempering were performed under the conditions shown in table 2 to produce seamless steel pipes. The preheating time in Table 2 is the residence time in the temperature range of 250 to 350 ℃. The quenching was performed by water quenching after high-frequency heating, and the cooling rate was adjusted to 150 ℃. The cooling at tempering was set to natural cooling in the atmosphere.

TABLE 2

For each of the obtained seamless steel pipes, first, the prior austenite grain size was measured. The prior austenite grain size is determined according to ASTM E112 (2013). Specifically, a test piece including the entire wall thickness was collected so that the longitudinal section of the seamless steel pipe was used as an observation surface, and mirror polishing of the observation surface was performed. After polishing, the prior austenite grain boundaries in the observation plane were developed using picric acid etching solution. Then, 5 visual fields were observed using an optical microscope so that the position 1/4 of the wall thickness from the outer surface of the seamless steel pipe was the center of the visual field. Then, the prior austenite grain size of each field was determined by the comparison method defined in ASTM E112 (2013), and the average value thereof was used as the prior austenite grain size of each seamless steel pipe. At this time, 100 times is taken as a reference observation magnification, and 200 times or 400 times is set depending on the grain size. When the observation magnification is 200 times or 400 times, correction is performed according to ASTM E112 (2013) using a correction value Q defined by the following formula (I).

Q=6.64log₁₀(M/100)···(I)

Wherein M in the above formula is the observation magnification.

Next, for each seamless steel pipe, mechanical properties, reducing workability, and hydrogen embrittlement resistance were evaluated by the following methods.

< Mechanical Properties >

A tubular test piece of a predetermined length was cut out from each seamless steel pipe, and a test piece No. 11 was produced in accordance with JIS Z2241:2011. Then, a tubular tensile test defined in JIS Z2241:2011 was performed using the test piece No. 11, whereby the tensile strength TS, the yield stress YS, and the elongation at break EL were measured.

< Reducing processability >

Each of 2 tubular test pieces having a length of 300mm was cut out from each seamless steel pipe, and then one end side of each tubular test piece was subjected to diameter reduction under conditions of a degree of working of 0.60 and 0.50, to form a reduced diameter portion having a length of 30 mm. The workability in the diameter reduction process is a value obtained by dividing the outer diameter of the reduced diameter portion by the outer diameter of the seamless steel pipe before the diameter reduction process. Then, the presence or absence of cracks at the reduced diameter portion was observed.

When no crack was generated under both conditions of the working degrees of 0.60 and 0.50, it was determined that the reducing workability was extremely Excellent (EX). When a crack was generated under the condition of a working degree of 0.50 but no crack was generated under the condition of a working degree of 0.60, it was determined that the reduced diameter workability was excellent (G). On the other hand, when cracks were generated under both conditions of the workability degree of 0.60 and 0.50, it was determined that the reducing workability (NA) was poor.

< Hydrogen embrittlement resistance Property >

An arc tensile test piece having the shape shown in fig. 1 was collected from each seamless steel pipe, and a cathode charging constant load test was performed. Specifically, a cathode charge constant load test was performed at a potential ranging from-0.9 to-1.2V while immersing a plurality of arc tensile test pieces each having a grip portion and a tenter portion in various aqueous solutions containing 3% NaCl and ammonium thiocyanate in a range of 0 to 30 g/L. At this time, a stress of 90% of the tensile strength of each seamless steel pipe was applied.

Then, only the test piece having a durability time exceeding 200 hours was stored in liquid nitrogen, and then the parallel portion of the tenter was cut off, and the hydrogen concentration was measured by a temperature programmed desorption hydrogen analysis method. In the temperature programmed desorption hydrogen analysis method, a test piece is heated to 200 ℃ from normal temperature at a temperature rise rate of 100 ℃ per hour, and then the amount of hydrogen released is measured, thereby obtaining the hydrogen concentration in the test piece. The maximum value of the obtained hydrogen concentrations was used as the limiting hydrogen concentration (Hc) and as an index of hydrogen embrittlement resistance. In this example, when Hc was 2.5ppm or more, it was judged that the hydrogen embrittlement resistance was excellent.

Table 3 shows the above evaluation results in a lump.

TABLE 3

TABLE 3 Table 3

As shown in table 3, test numbers 1 to 22 satisfying all the regulations of the present invention gave results that have high tensile strength and excellent reducing workability, and also excellent hydrogen embrittlement resistance. In contrast, test numbers 23 to 44 of comparative examples which did not satisfy the requirements of the present invention gave the results of deterioration of the reducing workability and hydrogen embrittlement resistance.

Industrial applicability

According to the present invention, a seamless steel pipe having high strength, excellent reducing workability, and excellent hydrogen embrittlement resistance can be obtained. Therefore, the seamless steel pipe of the present invention is suitable as a blank for an airbag inflator.

Claims

1. A seamless steel pipe,

Its chemical composition is calculated by mass% as C: 0.05-0.20%,

Si: 0.05-0.50%,

Mn: 0.30-1.50%,

P: 0.025% or less,

S: 0.020% or less,

Cu: 0.01～0.50%,

Ni: 0.01～0.50%,

Cr: 0.01～1.20%,

Mo: 0.01～0.50%,

Ti: 0.001～0.050%,

Nb: 0.001～0.100%,

Ca: 0.0005～0.0025%,

Al: 0.080% or less,

N: 0.0100% or less,

V: 0～0.100％

B: 0～0.0050%,

Mg: 0～0.0050%,

REM: 0～0.0050％

Sn: 0～0.100%,

As: 0～0.010%,

Balance: Fe and impurities,

Assuming that the content of each element is within the above range,

The relationship between the chemical composition and the wall thickness satisfies the following formula (i):

Furthermore, the relationship between the chemical composition and the prior austenite grain size satisfies the following formula (ii), the tensile strength is 1000 MPa or more,

Elongation at break is more than 8.0%,

The limit hydrogen concentration is 2.5ppm or above.

WT/(5C+Mo+Cr)≥1.00···(i)

GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

The element symbols in the above formula represent the content of each element in the steel in mass %, and are recorded as zero when not contained. In addition, WT represents the wall thickness of the seamless steel pipe in mm, and GN represents the original austenite grain size.

2. The seamless steel pipe according to claim 1, wherein

The chemical composition contains, in mass%,

V: 0.001～0.100%,

B: 0.0001～0.0050％,

Mg: 0.0001～0.0100%,

REM: 0.0001～0.0100％

Sn: 0.001～0.100%, and

As: one or more kinds of 0.001 to 0.010%.

3. An airbag inflator cylinder comprising:

The chemical composition of the cylindrical portion is C: 0.05-0.20% by mass,

Si: 0.05-0.50%,

Mn: 0.30-1.50%,

P: 0.025% or less,

S: 0.020% or less,

Cu: 0.01～0.50%,

Ni: 0.01～0.50%,

Cr: 0.01～1.20%,

Mo: 0.01～0.50%,

Ti: 0.001～0.050%,

Nb: 0.001～0.100%,

Ca: 0.0005～0.0025%,

Al: 0.080% or less,

N: 0.0100% or less,

V: 0～0.100％

B: 0～0.0050%,

Mg: 0～0.0050%,

REM: 0～0.0050％

Sn: 0～0.100%,

As: 0～0.010%,

Balance: Fe and impurities,

Assuming that the content of each element is within the above range,

The relationship between the chemical composition of the cylindrical portion and the wall thickness of the cylindrical portion satisfies the following formula (i):

Furthermore, the relationship between the chemical composition of the cylindrical portion and the prior austenite grain size of the cylindrical portion satisfies the following formula (ii):

The tensile strength of the cylindrical portion is 1000 MPa or more.

The elongation at break of the cylindrical portion is 8.0% or more,

The limit hydrogen concentration of the cylindrical portion is 2.5 ppm or more,

WT/(5C+Mo+Cr)≥1.00···(i)

GN-1.51×(Mn+85P-30Ca)≥8.50···(ii)

Among them, the element symbols in the above formula represent the content of each element in the steel of the cylindrical part, and the unit is mass %, and it is recorded as zero when it is not contained; in addition, WT represents the wall thickness of the cylindrical part, and the unit is mm, and GN represents the original austenite grain size of the cylindrical part.

4. The airbag inflator according to claim 3, wherein:

The chemical composition of the cylindrical portion contains, in mass %, a

V: 0.001～0.100%,

One or more of B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0100%, REM: 0.0001 to 0.0100%, Sn: 0.001 to 0.100%, and As: 0.001 to 0.010%.